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Can environmental factors acting on an organism cause inherited changes in phenotype in its descendants? It is now common knowledge that DNA is the principle hereditary material that determines an organisms phenotype and is what gets passed down to its descendants. This means that all the hereditary information is coded for by the DNA and that any thing the organism may do in their lifetime or any environmental change they may undergo will not be passed down to its offsprings. However, as molecular biology is advancing, some researchers have proved that factors other than changes in the genes can cause inherited changes in phenotype in an organisms descendants. Such an area of biology have been designated as Epigenetics. Epigenetics as understood in modern biology is “the study of heritable changes in gene function that occur without a change in the sequence of nuclear DNA”.1 this refers to any modification in the genes other than the change in the DNA sequences itself. It includes how environmental factors acting on an organism can change the way genes are expressed in the offspring. And how processes such as gene regulation are involved during the development of an organism. Gene regulation is when the level of gene expression can vary under different conditions. Each of our cells carry the genes which code for everything needed to sustain the body, but only some of them are active. This is because the epigenetic modifications act like switches, turning on the gene that is most needed in a particular cell at a particular time, so helping to control gene activity. In doing so, there is a memory of gene activity that can be passed on each time a cell divides. One of the well known epigenetic signal is DNA methylation which tags cytosine with a methyl group. It is generally associated with silencing gene expression because active genes are usually unmethylated. Another important epigenetic mechanism is chromatin remodelling. Chromatins are densely packed structure of coiled up DNA wrapped around proteins called histones inside the nucleus. These histones can be chemically modified by adding acetyle, methyl or phosphate groups thereby altering chromatin structure. This can then influence the activity of nearby genes, most probably switching them on. Together, these mechanisms regulate gene expression throughout the genome and may help to explain unusual occurrences such as parental ‘imprinting’. Parental or Genomic imprinting is an example of epigenetic inheritance. The word imprinting implies a type of marking process that has a memory. Similarly, genomic imprinting is when a segment of DNA is marked and that mark is retained and recognised throughout the life of the organism inheriting the marked DNA. The marking process causes the offspring to distinguish between maternally and paternally inherited alleles. This can sometimes lead to the offspring expressing one of the two alleles but not both, this process is known as monoallelic expression. At the molecular level, imprinting has been known to involve DNA known as Differentially Methylated Regions (DMRs) that are found near the imprinted genes. Depending on the type of genes, these regions are methylated in the oocyte or the sperm but not both. These DMRs contain binding sites for one or more proteins that regulate the transcription of nearby genes. Most probably, methylation at a DMR will result in an inhibition of gene expression. Due to this, imprinting is usually described as a marking process that silences gene expression by preventing transcription. Imprinting plays a key role in the inheritance of certain human diseases such as PraderWilli syndrome (PWS) and Angelman syndrome (AS). Patients suffering from PraderWilli syndrome have reduced motor function, obesity and mental deficiencies. Whereas Angelman syndrome patients are hyperactive, have unusual seizures and repetitive symmetrical muscle movements and show mental deficiencies. Both syndromes involve a small deletion in human chromosome 15 (locus 3), if this deletion is inherited from the maternal parent, it leads to Angelman syndrome, paternal inheritance leads to PraderWilli syndrome. Research has shown that this region contains closely linked but distinct genes that are maternally or paternally imprinted. Consequently, research in genomic imprinting as well as other epigenetic inheritance mechanisms not only show that genes have switches that can regulate gene expression but that these ‘switches’ can also be passed down to succeeding generations. This means that a 'memory' of an event could be inherited since a simple environmental effect could switch genes on or off . A recent finding has revealed that environmental factors affecting a particular generation can have consequences that can be passed down to a later generation. The research has shown that a famine that has occurred in the lives of the grandparents can cut short the life expectancy of their grandchildren. One such research2 was conducted in a German – imposed food embargo in western Holland during a famine in winter. They have linked prenatal exposure to famine to a range of developmental and adult disorders such as diabetes, obesity, coronary heart disease, cancer and birth of smaller-than-normal grand children. Such findings have led some people to claim “you are what your grandmother ate”3. such bold statements may come as a shock to the scientific community in the 21st century because it remind us of Lamarckian inheritance of acquired characteristics. The long refuted Lamarckian theory stated that when an organism acquired a certain characteristic in their lifetime, and he used the example of giraffes stretching their necks to reach treetops, they would give birth to progeny with similarly stretched necks. Although epigenetic inheritance isn’t necessarily bringing this theory back, but it is giving reason to reconsider how environmental cues could cause phenotypic changes transmittable to offspring."He had a basically good idea but a bad example," (Rohl Oflsson, Uppsala University, Sweden) Some scientists on the other hand, had alternative ideas to explain epigenesis. The psychologist Erik Erikson4 developed an epigenetic theory to explain human development on the basis of psycho-social crisis. He believed that every individual in their life goes through several developmental stages which are marked by a crisis. According to this theory, although these stages are largely predetermined by genetics, the way in which the crisis is overcome are not. In otherwords, the environmental factors are largely responsible for causing the changes in an organisms phenotype. Baldwinian evolution is another theory developed to explain how organisms can pass learned abilities to their offspring. It is said to be an intermediate between natural selection and social adaptation. It explains that as well as inheriting the genes encoded to by the DNA, through non specific selection the selected offspring would have an increased capacity for learning new skills as well as skills learned by the their ancestors. It may be a crucial factor for explaining the rapid evolution of the human mind which as many believe cannot simply have been the product of mere natural selection. Therefore, when it comes to unusual phenomena, such as when identical twins who share the same DNA and often the same environment can sometimes be so different ( for instance if one is diagnosed with schizophrenia while the other is fine), simple genetic can fail to give an answer. A better explanation can indeed be offered by epigeneticists who say that the differences between identical twins are due to changes in the genome that don’t effect the DNA sequences. Perhaps epigenetics is an adaptive mechanism through which humans and many other organisms may have adapted to change in the environment. It seems to provide a means by which organism can react to environmental ques. This can even be advantageous to the offspring since it enables it to adapt better to the environment. The exact extent to which epigenetics affects the phenotype of an organism may not still be known but what is for sure is that epigenetics is here to stay as more and more evidences accumulate to suggest that a simple DNA to RNA to PROTIENE association is not enough to explain heredity. Such evidence seem to prove that environmental factors acting on an organism can cause inherited changes in phenotype in its descendants. Word count: 1,341